1. Field of the Invention
The present invention relates to an ion source. More particularly, the present invention relates to a repeller structure configured to be mounted in a plasma generating chamber of an ion source. Such a repeller structure is typically arranged opposite to a cathode that emits electrons to repel the electrons toward the cathode.
2. Description of the Related Art
In recent years, a technique is considered in which, a source gas is ionized in a plasma generating chamber of an ion source by a cathode to generate a plasma and a sputtering target is sputtered by the plasma to cause desired ion species to be contained in an ion beam.
Specifically, as described in Japanese Patent Application Laid-open No. 2002-117780, a sputtering target provided at an end portion of a repeller is held in a replaceable manner to make it possible to generate stable ion species. The detailed structure includes a tubular repeller and a sputtering target (slug) that is contained in the end portion of the repeller. A step portion protruding inwards is provided on an inner peripheral surface of the end portion of the repeller, and a latch portion that latches the step portion is provided on an outer peripheral surface of the sputtering target. The sputtering target is fixed in the repeller by screwing a screw block that screws the sputtering target with a thread portion formed on the inner peripheral surface of the repeller from the upper part of the repeller in a state in which the latch portion of the sputtering target and the step portion of the repeller are latched together.
However, because the outer peripheral surface of the sputtering target is fixed by the repeller in a limited space in a plasma generating chamber, there is restriction in the size of the repeller. That also causes the size of the sputtering target to be contained in the repeller to be restricted, which leads to a problem that it is difficult to increase the surface area of the sputtering target.
There is another problem that a larger repeller must be used because the repeller is arranged on the outer peripheral surface of the sputtering target. Furthermore, because the thread portion is provided on the tubular inner surface of the repeller, not only the structure of the repeller becomes complicated, but also material cost and processing cost could increase if the repeller is manufactured by machining a single workpiece.
In addition, because the repeller is arranged on the outer circumference of the sputtering target with respect to electrons emitted from the cathode, a member facing a portion from where the electrons are emitted from the cathode becomes the sputtering target, resulting in a problem that the electron reflection efficiency is degraded, and as a result, the plasma generation efficiency is degraded.